Our research deals with the structure, assembly and function of microtubules, which are involved in chromosome segregation, cell motility, the maintenance of cell shape, and the organization of cell surfaces. In the past year we have been studying a post-translational modification of tubulin (the protein building block of microtubules) which we think is an excellent candidate to regulate microtubule assembly or function. This involves the reversible enzymatic addition of a tyrosine residue to the C-terminus of the alpha-chain of tubulin. We are characterizing the enzymes, substrates and reactions involved. We are also studying tubulin modification in cultured neuronal cells in vivo, and have some evidence that brain membrane tubulin is less tyrosylated than cytoplasmic tubulin. BIBLIOGRAPHIC REFERENCES: Raybin, D. and Flavin, M.: Modification of tubulin by tyrosylation in cells and extracts and its effects on assembly in vitro. J. Cell Biol. 73: 492-504, 1977. Raybin, D. and Flavin, M.: An enzyme which specifically adds tyrosine to the alpha-chain of tubulin. Biochemistry 16: 2189-2194, 1977.